The present invention relates to a distance measuring apparatus and, more particularly, to a distance measuring apparatus used for, e.g., measuring the distance to a target object (object to be photographed).
In general, distance measuring apparatuses have conventionally used the principle of trigonometric distance measurement.
Such distance measuring apparatuses can be classified into an active scheme type using reflected light upon projecting distance measurement light, and a passive scheme type using the correlation between the luminance distributions of a target object when viewed from two points.
Both types of distance measuring apparatuses are employed for many cameras.
However, a distance measuring apparatus of the active scheme type cannot reliably measure distance when the target object is located farther than the projected distance measurement light can reach.
A distance measuring apparatus of the passive scheme type, moreover, cannot reliably measure distance because of an out-of-focus state which occur when the luminance difference between the luminance distributions of the target object is small, i.e., the contrast is low, or when the contrast of objects except for the main target object is high, e.g., when a person stands with a night scene in the background.
To compensate for the weak points of the two schemes, a hybrid distance measuring apparatus using a combination of the two schemes is proposed and well known.
If, however, the active and passive schemes are simply used, the above-described hybrid distance measuring apparatus using a combination of the two schemes requires a high manufacturing cost and becomes bulky.
Even if the hybrid distance measuring apparatus switches these schemes using a common sensor by elaborating the circuit, a specific, complicated circuit is necessary, resulting in a high-cost apparatus though it can be small.
In addition, there is a well-known distance measuring apparatus using a scheme of compensating for the weak point of the passive scheme by projecting auxiliary light when the target object is dark, or when the luminance difference between the luminance distributions of the target object is small and the contrast is low.
The distance measuring apparatus of this type, however, cannot supply a sufficient light quantity when the target object is at a far distance, or when surroundings are bright and the contrast is low. This distance measuring apparatus is effectively used only when the target object is at a near distance.
When the light quantity of reflected light from the target object is small, auxiliary light may be projected for a long time and photocurrents of the reflected light may be integrated. However, projection of extra auxiliary light leads to energy waste.